In known breath alcohol measuring apparatus, the alcohol concentration in the exhaled air is measured with indirect measuring methods, for example, via the electrochemical conversion of the alcohol molecules in a measuring cell, by infrared absorption or via the change of conductivity of a semiconductor. A conclusion as to the blood alcohol concentration can be drawn via empirical models of the and equilibrium and the kinetic of the mass transfer between the alcohol content of the blood and the alcohol concentration in the gas phase. The temperature of the gas phase and the temperature of the blood participate decisively in these empirical models. For example, the concentration of ethanol in the gas phase above an aqueous solution of ethanol changes by more than 6% per degree Kelvin.
In the context of field tests, measurements of test persons were carried out and differences in the respiratory gas temperature of up to 5.8 degrees Kelvin were determined. Such a temperature difference corresponds to a relative change of the equilibrium concentration of ethanol in the exhaled air of almost 40%. In known breath alcohol measuring apparatus, the temperature of the respiratory gas sample is measured in order to detect the fluctuations of the temperature of the respiratory gas and this temperature is used for corrective purposes.
A breath alcohol measuring apparatus is disclosed in the publication of G. Schoknecht et al entitled xe2x80x9cThe Technical Concept for Evidential Breath Testing in Germanyxe2x80x9d, 13th International Conference on Alcohol, Drugs and Traffic Safety, Adelaide, Australia, Aug. 13 to 18, 1995. In the known breath alcohol measuring apparatus, a mouthpiece is seated in a preheated holder and a temperature sensor is disposed in the sample intake channel which is enclosed by the wall of the holder. Thermal elements or resistance temperature sensors are usually used as temperature sensors. The respiratory gas sample travels via the mouthpiece and a sample intake tube to a measuring chamber of an infrared measuring device.
It is disadvantageous in the known breath alcohol measuring apparatus that the respiratory gas sample exchanges heat with the mouthpiece at the location of the temperature measurement and this can lead to a change of the temperature measurement value by several degrees Kelvin. Especially large changes result when cold mouthpieces are used or when the construction of the mouthpiece is unfavorable, for example, with respect to material or wall thickness.
It is an object of the invention to provide a breath alcohol measuring apparatus of the type described above which is improved so that the influence of the mouthpiece on the measurement of temperature of the respiratory gas sample is substantially eliminated.
The breath alcohol measuring apparatus of the invention includes: a housing defining a sample intake channel for receiving a sample of respiratory gas from a test person; a filter disposed in the sample intake channel and being transmissive for infrared radiation emanating from the test person; and, a temperature sensor mounted downstream of the filter and the temperature sensor being an infrared optical thermometer.
The advantage of the invention is essentially that an infrared optical thermometer having a filter is used as the temperature sensor. The filter is permeable for infrared radiation and is mounted in the sample intake channel. The infrared optical thermometer evaluates the infrared radiation passing through the filter. The filter is positioned within the sample intake channel so that the infrared radiation, which is emitted from the mouth and/or throat area of the test person, is detected and thereby also the temperature of the oral cavity. The temperature of the oral cavity is significantly better suited for correcting the influence of temperature on the breath alcohol measurement than the temperature of the respiratory gas in the sample intake channel because the mass transfer between the respiratory air and the body tissue of the test person takes place almost exclusively on the path from the lung into the mouth region. The infrared radiation can be coupled out of the sample intake channel behind the filter with a simple collecting optic and be evaluated by the infrared optical thermometer. The infrared radiation path is so configured that no infrared radiation, which is reflected within the sample intake channel, reaches the thermometer. With the temperature measurement according to the invention, measuring accuracies in the order of magnitude of approximately 0.2 Kelvin are realized.
It is especially advantageous to arrange the filter within the sample intake channel perpendicular to the flow direction. Especially good temperature measuring results are achieved when the filter is located in the vicinity of the test person.
It is especially advantageous to place the filter directly in a mouthpiece connectable to the sample intake channel. For this purpose, the mouthpiece has a tubular inner part around which the respiratory gas sample flows. This inner part is closed by a wall piece formed as one piece on the inner part and is configured as a filter. With an appropriate selection of material, the mouthpiece as well as the filter can be injection molded from the same material such as polyethylene. Many other plastics can be used which are transmissive for infrared radiation.
An especially advantageous embodiment of the invention is to configure a membrane of a check valve, which is located in the mouthpiece, as a filter permeable for the infrared radiation. Check valves of this kind serve to prevent the test person from re-inhaling air already exhaled into the sample intake tube and are mostly anyway present so that no additional parts are needed. As a membrane, a material is selected which is permeable for infrared radiation in the wavelength range of interest.
The membrane of the check valve is configured as an infrared filter and can replace the filter located in the tubular inner part or can be provided additionally to this filter.
The infrared radiation passing through the filter is deflected onto the infrared optical thermometer by an aspherical mirror mounted in the sample intake channel.